remove vm session feature

Cuts the daemon-managed guest-session machinery (start/list/show/
logs/stop/kill/attach/send). The feature shipped aimed at agent-
orchestration workflows (programmatic stdin piping into a long-lived
guest process) that aren't driving any concrete user today, and the
~2.3K LOC of daemon surface area — attach bridge, FIFO keepalive,
controller registry, sessionstream framing, SQLite persistence — was
locking in an API we'd have to keep through v0.1.0.

Anything session-flavoured that people actually need today can be
done with `vm ssh + tmux` or `vm run -- cmd`.

Deleted:
- internal/cli/commands_vm_session.go
- internal/daemon/{guest_sessions,session_lifecycle,session_attach,session_stream,session_controller}.go
- internal/daemon/session/ (guest-session helpers package)
- internal/sessionstream/ (framing package)
- internal/daemon/guest_sessions_test.go
- internal/store/guest_session_test.go
- GuestSession* types from internal/{api,model}
- Store UpsertGuestSession/GetGuestSession/ListGuestSessionsByVM/DeleteGuestSession + scanner helpers
- guest.session.* RPC dispatch entries
- 5 CLI session tests, 2 completion tests, 2 printer tests

Extracted:
- ShellQuote + FormatStepError lifted to internal/daemon/workspace/util.go
  (only non-session consumer); workspace package now self-contained
- internal/daemon/guest_ssh.go keeps guestSSHClient + dialGuest +
  waitForGuestSSH — still used by workspace prepare/export
- internal/daemon/fake_firecracker_test.go preserves the test helper
  that used to live in guest_sessions_test.go

Store schema: CREATE TABLE guest_sessions and its column migrations
removed. Existing dev DBs keep an orphan table (harmless, pre-v0.1.0).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/cli/completion.go

@@ -21,9 +21,9 @@ import (
 //   - Fail silently. Completion is advisory; any error path returns an
 //     empty suggestion list rather than propagating to the user.
 
-// defaultCompletionLister + defaultCompletionSessionLister back the
-// corresponding *deps fields; tests inject their own fakes via the
-// struct instead of mutating package-level vars.
+// defaultCompletionLister backs the *deps.completionLister field;
+// tests inject their own fake via the struct instead of mutating
+// package-level vars.
 func defaultCompletionLister(ctx context.Context, socketPath, method string) ([]string, error) {
 	switch method {
 	case "vm.list":
@@ -66,20 +66,6 @@ func defaultCompletionLister(ctx context.Context, socketPath, method string) ([]
 	return nil, nil
 }
 
-func defaultCompletionSessionLister(ctx context.Context, socketPath, vmIDOrName string) ([]string, error) {
-	result, err := rpc.Call[api.GuestSessionListResult](ctx, socketPath, "guest.session.list", api.VMRefParams{IDOrName: vmIDOrName})
-	if err != nil {
-		return nil, err
-	}
-	names := make([]string, 0, len(result.Sessions))
-	for _, session := range result.Sessions {
-		if session.Name != "" {
-			names = append(names, session.Name)
-		}
-	}
-	return names, nil
-}
-
 // daemonSocketForCompletion returns the socket path IFF the daemon is
 // already running. Returns "", false when no daemon is up — completion
 // callers use this as the bail signal.
@@ -177,25 +163,3 @@ func (d *deps) completeKernelNames(cmd *cobra.Command, args []string, toComplete
 	}
 	return filterPrefix(names, args, toComplete), cobra.ShellCompDirectiveNoFileComp
 }
-
-// completeSessionNames handles `... <vm> <session>` commands: pos 0
-// completes VMs, pos 1 completes sessions owned by args[0], pos 2+ is
-// silent.
-func (d *deps) completeSessionNames(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
-	switch len(args) {
-	case 0:
-		return d.completeVMNames(cmd, args, toComplete)
-	case 1:
-		socket, ok := d.daemonSocketForCompletion(cmd.Context())
-		if !ok {
-			return nil, cobra.ShellCompDirectiveNoFileComp
-		}
-		names, err := d.completionSessionLister(cmd.Context(), socket, args[0])
-		if err != nil {
-			return nil, cobra.ShellCompDirectiveNoFileComp
-		}
-		return filterPrefix(names, nil, toComplete), cobra.ShellCompDirectiveNoFileComp
-	default:
-		return nil, cobra.ShellCompDirectiveNoFileComp
-	}
-}